The invention relates to a method for time coordination of the transmission of cyclic data values on a bus to which data transmitters, which cyclically repeatedly transmit a predetermined number of data values, and data receivers are connected.
Digital devices are used for serial transmission of data in technical fields. For example, this applies to data transmission in motor vehicles and in measurement and control engineering for information interchange between measurement devices (data transmitters) and controllers (data receivers). The bus used in this case is a so-called CSMA/CA bus, that is to say a bus referred to in the specialist literature as a Carrier Sense Multiple Access/Collision-Avoidance bus. A bus which generally has two conductors is used for serial data transmission. During typical operation of the bus, a large amount of data is transmitted cyclically from the measurement devices to the controllers. Apart from measurement devices, actuators such as pumps, valves and the like may also be connected to the bus. With the present bus standard, there is no control of the timing of accesses by individual bus subscribers to the bus. Each subscriber just waits for a minimum time when the bus is quiet, and transmits its data message once this time has elapsed. This can lead to so-called xe2x80x9cburstsxe2x80x9d, that is to say a very large number of messages are transmitted in a very short time. Such a flood of data can lead to the processor modules in some bus subscribers being overloaded. In an extreme situation, this can even lead to loss of data, since received messages are not received.
The invention is based on the object of specifying a method using which data values can be transmitted in a time-coordinated manner while avoiding overload situations.
This object is achieved according to the invention by a method for time coordination of the transmission of cyclic data values on a bus to which data transmitters, which cyclically repeatedly transmit a predetermined number of data values, and data receivers are connected, in which
a) each data transmitter is assigned a cycle time in which it periodically transmits its data items, wherein the cycle times are integer multiples of a minimum cycle time,
b) each data value is assigned a delay time which is related to the start of the cycle time of its data transmitter,
c) a synchronization message is transmitted via the bus to each data transmitter with a period corresponding to the cycle time of the respective data transmitter and has a phase which is specific for the data transmitter with respect to the start of the minimum cycle time, and the reception of its synchronization message in each data transmitter initiates the transmission of its data values with the respective delay time, and
d) monitoring times which are dependent on cycle times are defined in each data receiver, in which monitoring times the data receivers listen to the bus in order to receive the transmitted data values.
The use of the method according to the invention avoids the data transmitters starting to transmit their data values to the bus virtually at the same time. Instead of this, the data values are transmitted via the bus with a time delay corresponding to the associated cycle and delay times and corresponding to the phase of the respective cycle time with respect to the minimum cycle time, so that said so-called bursts can no longer occur which, although not regarded as a fault situation according to the currently applicable standard, nevertheless can lead to an excessive bus load, and in some circumstances even to loss of data.
Advantageous developments of the invention are described in the dependent claims.